Overall my rating of Sherlock Holmes is a solid positive, eight, maybe eight and a half, stars out of ten. There's nothing wrong with it; if I list the elements and try to point at a "this could be better" or "this was missing" I don't find any flaws to highlight. The movie has everything and seems to have done a good job of putting it together. But somehow it didn't quite click into being an "ohmygod I want to watch it again!" movie.
I don't personally get hung up on the whole "Jeremy Brett or no one" thing that some people stick to. While I'm happy to compare one portrayal to another I never felt like one precludes the other, particularly when they're different stories -- watching Downey redo the classic Doyle stories would invite a lot of comparison to Brett (or your other favorite Holmes, whoever that is), admittedly, but a new story featuring familiar characters necessitates less comparison. Downey's roguish Holmes isn't quite Brett's clipped and fastidious one (though Downey's not nearly as roguish as the trailer wants you to think) but I think both of them are consistent with the stories to a reasonable and acceptable extent. (Though deviations from the books can also be fun to me. I've never been the kind of purist who would turn my nose up at the fun of Without A Clue, for instance.)
The trailer does tend to give a misleading impression in several ways and I'm happy to note that the movie did not succumb to the red flags in the trailer. The groaningly awful innuendo designed to play up the rascal side of Holmes is cherry-picked out of a lot of other stuff that shows the rest of Holmes, giving a false impression that hinted at a very shallow Holmes aimed at a least-common-denominator audience. The trailer made Watson out as a cranky second-rate foil to an extent that is not borne out by the movie, where he is fleshed out and sympathetic. The storyline promises to go in directions that don't fit Doyle's Holmes stories but (without spoiling anything) ultimately plays out as within the realm of Holmes (though with a few liberties taken with some of the familiar characters).
One thing that worried me in the trailers were the fight scenes. Not the idea of Holmes fighting -- while Holmes doesn't box in the stories, there are references to him having boxed and been good at it, and also having learned some Eastern martial arts, so it's quite acceptable. Rather, that thing where they switch between regular and slow-motion back and forth in the middle of a single movement, which has been trendy and annoying of late. For once, this technique is not egregious but put to a real, and effective, purpose in the movie, and we see it within the first couple of minutes. In fact, it turns out to be a brilliant solution to the problem of conveying the techniques Holmes is using as he fights.
For two hours twenty minutes, the movie crams a lot of story, a lot of mysteries to be solved, a lot of challenges to be overcome. There's enough here to make two movies easily (and I don't just mean the intentionally dangling threads intended to set up future movies -- just the storylines of this movie could have made two). A few of the mysteries have resolutions that feel a little pat, but only a little. Most of them tie up excellently neatly and in a very Holmesian manner (apart from there being a lot more action than in a typical Doyle story).
I wish I could put my finger on why the movie doesn't quite rise the last few notches from "very good" to "fantastic". Ultimately, I would certainly recommend it to any fans of either the mystery or action genre, but I wouldn't put it on the "must see on the big screen" list -- seeing it at my house will be perfectly adequate.